Fuel cells represent a non-polluting and alternative energy source to hydrocarbon combustion, particularly for motor vehicles. A fuel cell is a battery in which the electricity is generated by the oxidation of a reducing fuel, for example hydrogen, on an electrode, coupled with the reduction of an oxidizer, such as oxygen in the air, on the other electrode. The hydrogen oxidation reaction can be accelerated by using a catalyst which generally contains a metallic element.
However, in the case of fuel cells using hydrogen, an extremely inflammable product, one of the main obstacles is the production and/or storage of this fuel. This is why technological decisions today tend to favour liquid fuels that are directly oxidized at the anode of the cell, or liquid or solid fuels capable of generating hydrogen “as required”, that is, whereby the quantity of hydrogen generated equals the quantity of hydrogen consumed by the cell.
One of the known methods for producing hydrogen is to hydrolyze borohydrides in solid form, like sodium borohydride or tetrahydroborate NaBH4, dissolved and contacted with a solid catalyst material. The sodium borohydride is hydrolyzed under certain conditions by the following reaction:M(BH4)n+2nH2O→M[B(OH)4]n+4nH2 where M is an alkali or alkaline earth element and n is a positive whole number equal to the number of valency electrons of the element M. The element M may for example be sodium (Na), in which case the number n is 1, giving rise to the following reaction:NaBH4+2H2O→NaB(OH)4+4H2.
However, the element M may be potassium (K), lithium (Li) or another appropriate element.
This borohydride hydrolysis reaction has the advantage of involving reagents and residues which are harmless, that is, non-toxic and non-polluting, as opposed for example to the reactions occurring in the Direct Methanol Fuel Cell (DMFC) or Formic Acid Fuel Cell (FAFC) fuel cells, which use methanol and formic acid respectively as fuel.
According to this reaction scheme, the commonly used sodium borohydride generates four moles of hydrogen by reacting with two moles of water.
In order to improve the yield of the hydrolysis reaction, some prior art reactors use borohydrides in solid form, for example in the divided state, that is in powder form. The hydrogen is then generated by contacting the solid borohydride with an aqueous solution preferably containing a catalyst material.
However, this type of hydrogen generator has drawbacks which complicate its use. It is difficult to control the reaction between the solid borohydride and an aqueous solution, and hence the flow of hydrogen generated.
In fact, this heterogeneous reaction, that is involving reagents in the liquid and solid states, is difficult to control, because it requires mechanisms for diffusing the water towards the borohydride.
These diffusion mechanisms depend strongly on the porosity of the reaction medium, which varies with the advancement of the hydrolysis reaction because of the formation of more or less compact by-products. This makes it difficult to control the kinetics of the hydrolysis reaction in the case of the devices described by documents US-2002-182459, WO-A-2005/102914, U.S. Pat. No. 4,261,956, U.S. Pat. No. 4,261,955, which provide for contacting the reagents by diffusion across a porous membrane. Such a membrane is in fact subject to obstruction by the compact reaction products, such as the compound NaB(OH)4.
Furthermore, these reaction by-products frequently obstruct the water intake line in the reactor containing the solid borohydride. In consequence, the reaction kinetics is liable to fluctuate according to the extent of this obstruction of the water intake. To avoid the addition of heavy and costly equipment to solve this problem, it is necessary to optimize the hydrogen generation by maximum control of the reaction.
Documents WO02/30810 and US2001/045364 also describe hydrogen generators in which the hydrolysis reactions are inaccurately controlled, because the reagents are contacted at several points, so that the reactions are all initiated and take place almost simultaneously over a large reaction exchange area.